This invention relates to an image intensifier camera which has automatic exposure control and which has application in aerial and ground surveillance photography.
The typical image intensifier camera consists, basically, of an image intensifier tube having a photocathode, an anode, and an intensifier phosphor screen, an objective lens to focus light from a given scene onto the photocathode, an aperture and a shutter to control the amount and the duration of the focused light striking the photocathode, and a light sensitive film on which an image is recorded when the film is struck by intensified light from the intensifier phosphor screen.
Currently, three types of image intensifier tubes are used in such cameras. The first type is an electrostatically focused image intensifier tube. This tube consists of an electrode located between the photocathode and anode. The electrode is used as a gating and focusing device. The second type is an electromagnetically focused image intensifier tube. External to the tube is a focusing solenoid or focusing magnet which is located parallel to and midway between the photocathode and the anode, and which produces a magnetic field to accomplish focusing. The third type is a micro- channel image intensifier tube. This tube consists of discrete channels located perpendicular to and between the photocathode and the anode. Focusing is accomplished by passage of electrons through the discrete channels.
In using image intensifier cameras for aerial and ground surveillance applications, there exists a need for rapid access to the recorded information because the photographic transparencies developed on the scene are interpreted prior to printing. This requires that there be constant tonal fidelity in the developed transparencies. There, therefore, exists a need for highly accurate automatic exposure control devices for rapid access photographic interpretation. With such accurate control, a roll of film can be exposed uniformly throughout a wide range of scene luminance and the uniform exposure coupled with uniform processing, can yield the tonal fidelity required in the resultant transparencies for rapid access photographic interpretation.
The known conventional techniques for automatic exposure control in image intensifier cameras employ a light sensor located external or internal to the camera body. The information from the light sensor is integrated through circuitry and is used to control exposure by regulating tube voltage, by regulating electronic gain width, by mechanically regulating an aperture, or by mechanically regulating a shutter. These conventional systems normally sense the light from a scene and electronically or mechanically preset the adjustable variables before exposing the film. Because of the response time involved in the electrical or mechanical adjustment and re-adjustment of the variables, a drastic change in scene illumination during exposure will cause either overexposure or underexposure of a given scene as recorded on the film.